In a mobile communication system, a situation may occur in which received power of a terminal communicating with a certain cell decreases due to movement of the terminal itself, change in the surrounding environment and/or the like, making it difficult to keep the communication. For such a situation, the terminal can maintain communication by reconnecting to a cell having higher received power than the cell in communication. The processing to switch between cells is referred to as “handover.” In order to perform handover, the terminal needs to measure the received power of a signal transmitted from a cell around a “serving cell” and report the received power to the serving cell. The term “serving cell” as used herein refers to a cell which notifies a terminal in communication of control information. In LTE, this processing performed by a terminal to measure the received power and/or reception quality of signals transmitted from surrounding cells (also referred to as neighboring cells”) is referred to as “measurement.”
Non-Patent Literature (hereinafter, referred to as “NPL”) 1 describes measurement and the operation of handover in LTE.
That is, a base station (serving cell) first configures a terminal in communication with respect to measurement. Specifically, the measurement configuration refers to setting of a measurement target (carrier frequency or the like), setting of a list of serving cell and neighboring cells (list of cell IDs), setting of measurement timing, and setting of details about report values. Setting of details about report values indicates setting of types of report values (Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) defined in NPL 2, and/or the like), setting of a trigger condition for prompting the base station to transmit report values, and setting of reporting periods. The terminal performs measurement using a reference signal (RS) generated using a cell-specific sequence, a synchronization signal, and a data signal transmitted from a serving cell and neighboring cells.
The terminal performs quality measurement of signals transmitted from the serving cell and neighboring cells on the basis of the setting by the base station. If a predetermined trigger condition set by the base station is satisfied, the terminal reports the measured value of quality to the base station (i.e., serving cell). A trigger condition is, for example, a condition that the quality level of the serving cell exceeds a predetermined threshold, or a condition that the quality of a neighboring cell exceeds the quality of the serving cell by a predetermined threshold or more.
The base station determines a handover destination cell based on the measured value of quality reported from the terminal and instructs the terminal to perform handover. Upon receiving the instruction from the base station, the terminal switches from the serving cell to another by disconnecting communication with the handover source cell and connecting to the handover destination cell.
In addition, studies have been carried out on CoMP (Coordinated multiple point transmission and reception), a communication system in which a plurality of cells transmits data to a terminal in a coordinated manner, mainly for the purpose of improving throughput for users at the cell edge in the downlink of LTE-Advanced, which is an evolved form of 3GPP LTE (3rd Generation Partnership Project Long-Term Evolution).
In LTE-Advanced, Dynamic Cell Selection (DCS) and Joint Transmission (JT) have been under study as a CoMP scheme. DCS is a system that dynamically selects a cell transmitting data according to the propagation state from among a plurality of cells involved in CoMP. In addition, JT is a system in which a plurality of cells involved in CoMP transmits the same signal simultaneously to terminals.
In order to improve the transmission characteristics by CoMP at a lower cost, studies have been carried out on forming a plurality of cells with Baseband Signal Processors (BSP) and Remote Radio Heads (RRH). FIG. 1 shows two cell groups (CoMP groups in the drawing) each including three RRHs and one BSP. Then, each cell corresponds to one of groups of pairs each formed of each RRH and one BSP.
A BSP is a processor which performs baseband signal processing such as generation of transmission signals to a terminal, demodulation of signals received from a terminal, and scheduling of a plurality of terminals. The BSP is connected to a plurality of RRHs with a cable (optical fiber) and communicates with terminals via the RRHs. Note that, the BSP may also be referred to as a Base Band Unit (BBU).
An RRH includes an amplifier and an antenna. On the downlink, the RRH receives as input a baseband signal generated in the BSP and transmits, to the terminal via an antenna, a signal obtained by performing transmission processing on the received baseband signal, such as D/A conversion, up-conversion, amplification, and/or the like. On the other hand, on the uplink, the RRH transmits, to the BSP, a baseband signal obtained by performing reception processing on the signal received from the terminal via the antenna, such as down-conversion, A/D conversion, and/or the like. Note that, an RRH, also referred to as a “remote base station,” is recognized by a terminal as a cell.
As shown in FIG. 1, applying CoMP only between a plurality of RRHs (cells) connected to a single BSP has been under study in LTE-Advanced. In other words, no CoMP is applied between RRHs which are connected to different BSPs. This is because only a small performance advantage is expected by CoMP. In addition, performing CoMP of this type requires a new I/F to be defined between BSPs, which results in a further increased control delay. The configuration in which a single BSP and a plurality of RRHs connected to the BSP is referred to as a “CoMP group.” Note that, a CoMP group may also be referred to as a CoMP cooperating set.